The ethylene/propylene/diene terpolymer rubbers (such as EPDM and EPT) have no double bond in the principal chain of its molecular structure, so that they are highly resistant to weather, ozone and heat aging although the oil resistance thereof is poor. Thus, because of their excellent properties, they are often employed in automobile parts, at locations to which static force applies, such as a weather stripping, a door glass run channel and a radiator hose.
On the other hand, the nitrile rubber (NBR) has excellent strength characteristics and oil resistance although the heat resistance and low temperature flexibility thereof are poor, so that it is often employed in a hose and a packing arranged around an engine.
Most parts requiring mechanical strength against dynamic fatigue, such as a tire and a rubber vibration insulator, are composed of a diene rubber, for example, NR, SBR or BR or a blend thereof.
There are a number of automobile parts required to simultaneously have the excellent properties of both polymers of EPDM and NBR, which would be realized by improving the oil resistance of EPDM or improving the heat resistance and low temperature flexibility of NBR.
For obtaining a homogeneous compound by blending EPDM and NBR, first, it is needed to take measures for coping with the difference in polarity between these polymers.
Such measures are being studied from the angle of milling procedure. For example, the attempt to render the compound dispersibility uniform by adding a large amount of carbon black to both polymers of EPDM and NBR to thereby prepare a carbon master batch (CMB) is reported in J. M. Mitchl: Rubber Chem. Technol., 50, 430 (1977).
Further, the attempt to uniformly disperse the vulcanization accelerator by the use of Pb.sub.3 O.sub.4 is reported in W. H. Whttington; Rubber Ind., 9, 151 (1975).
Secondly, it is needed to take measures for improving the covulcanizability of diene polymers NBR and EPDM.
Such measures include the attempt to improve the above covulcanizability by increasing the vulcanization velocity of EPDM which is generally very low. In this attempt, EPDM is reacted with sulfur in advance to thereby increase the vulcanization velocity of EPDM. This is well known as the pendant sulfur method.
Blend compounds of NBR and EPDM having been prepared by the partial or full utilization of such measures are now commercially available.
Although the problem of polymer polarity can be alleviated to some extent by the above measures, however, the vulcanization velocity of EPDM is conspicuously lower than that of NBR, so that the covulcanizability of EPDM/NBR blend compounds is not always satisfactory.
The attempt to increase the vulcanization velocity of EPDM is described in, for example, Japanese Patent Laid-open Publication No. 6(1994)-128427. In this attempt, ethylene, propylene and 7-methyl-1,6-octadiene as a nonconjugated polyene are terpolymerized. The resultant EPDM exhibits a greatly increased vulcanization velocity, so that the covulcanizability with NBR is improved. That the covulcanizability of EPDM and NBR is improved by increasing the vulcanization velocity of EPDM is described in the publication.
However, it still cannot be stated that the covulcanizability of EPDM and NBR is satisfactorily high, and there is a demand for the development of a polymer such as a novel diene-bearing EPDM capable of increasing the vulcanization velocity to thereby further improve the covulcanizability with nitrile rubber (NBR).
Therefore, there is a demand for the developments of such a polymer, an ethylene/.alpha.-olefin/nonconjugated polyene terpolymer rubber composition composed of the above polymer and nitrile rubber (NBR), which composition enables providing a vulcanized rubber being excellent in not only strength characteristics and oil resistance but also resistances to weather, ozone and heat aging, and a vulcanized rubber obtained from the ethylene/.alpha.-olefin/nonconjugated polyene terpolymer rubber composition.
On the other hand, the improvement of the heat aging and weather resistances of automobile parts is demanded in accordance with the recent enhancement of the quality of the parts.
Although EPDM has excellent resistances to weather, ozone and heat aging, the use of EPDM alone in a tire or a rubber vibration insulator is not suitable because of its poor dynamic fatigue resistance.
Thus, blends of EPDM and a diene rubber have extensively been investigated for attaining the full exertion of the advantages of the materials. However, any EPDM/diene rubber blend being excellent in covulcanizability has not been obtained to thereby render its practical use unattainable.
With respect to the above investigations, the developed technologies on EPDM/diene rubber blends are summarized by Messrs. Yasuhiro Oda and Masashi Aoshima in Journal of the Society of Rubber Industry, Japan, 51, 685 (1978), which include (1) polysulfide vulcanization, (2) peroxide vulcanization, (3) application of preliminarily vulcanized EPDM, (4) application of EPDM of high iodine value, (5) application of halogenated EPDM and (6) use of an accelerator having a long-chain alkyl group.
Further, the above literature describes that EPDM having an intrinsic viscosity of at least 3.0 dl/g as measured in xylene at 70.degree. C., a propylene content of up to 35% and a high iodine value is preferred.
However, although the description of the literature suggests the direction of improvement of the covulcanizability of EPDM and a diene rubber, there is no description as to actual products, especially, product quality items needed when dynamic strength is required and the above technologies cannot realize the properties desired in the present invention at all.
The object of blending EPDM with a diene rubber resides in the realization of excellent heat aging and weather resistances without practical deterioration of the excellent abrasion, dynamic fatigue and crack growth resistances possessed by the diene rubber. Thus, the EPDM to be blended with the diene rubber must also be provided with dynamic fatigue resistance.
Rubber Chemistry Technology, vol. 44, October 1971, p1043 mentioned a rubber vibration insulator material as a member most required to possess dynamic fatigue resistance, and it is described there that the use of a high molecular weight EPDM of high Mooney viscosity as a rubber vibration insulator material enables obtaining a vulcanized rubber being excellent in dynamic fatigue resistance.
However, the use of a high molecular weight EPDM would be thought of by any one skilled in the art to which the present invention pertains. The matter to which the most intensive research and development efforts should be directed is to improve the covulcanizability with a diene rubber which has been the drawback of conventional EPDM.
With respect to the method of improving the covulcanizability of EPDM and a diene rubber, it has been attempted to effect a terpolymerization of ethylene and propylene with each of new dienes of high vulcanization velocity in place of the conventional ethylidenenorbornene.
However, the covulcanizability with a diene rubber is still unsatisfactory with respect to the EPDMs obtained by the attempted terpolymerization using such new dienes.
Therefore, there is a demand for the developments of an ethylene/.alpha.-olefin/nonconjugated polyene terpolymer rubber whose vulcanization velocity is on the same level as that of a diene rubber, and an ethylene/.alpha.-olefin/nonconjugated polyene terpolymer rubber composition which enables providing a vulcanized rubber having excellent resistances to weather, ozone and heat aging without detriment to the excellent mechanical properties, abrasion resistance and dynamic fatigue resistance possessed by the diene rubber and a vulcanized rubber obtained from the composition.
As mentioned above, EPDM has no unsaturated bond in the principal chain of its molecular structure, so that it has excellent resistances to weather, ozone and heat aging as compared with those of the customary diene rubbers. Consequently, EPDM is widely used in, for example, rubber parts for electrical and electronic appliances and for civil work and construction.
Examples of suitable EPDMs include ethylene/propylene/dicyclopentadiene, ethylene/propylene/1,4-hexadiene and ethylene/propylene/5-ethylidene-2-norbornene terpolymer rubbers. Of these, ethylene/propylene/5-ethylidene-2-norbornene terpolymer rubber whose vulcanization velocity is high is especially widely employed.
However, even the ethylene/propylene/5-ethylidene-2-norbornene terpolymer rubber has the drawback that its vulcanization velocity is low as compared with that of a diene rubber such as natural rubber (NR), styrene/butadiene rubber (SBR), butadiene rubber (BR) or nitrile rubber (NBR) and that its covulcanizability with a diene rubber is poor. Further, with respect to the ethylene/propylene/5-ethylidene-2-norbornene terpolymer rubber, it is difficult to shorten the vulcanization time for enhancing the productivity of vulcanized rubber products or to lower the vulcanization temperature for reducing the energy consumption during the vulcanization.
The velocity of vulcanization of the conventional EPDM can be increased by increasing the amounts of vulcanizer and vulcanization accelerator. However, the use of vulcanizer and vulcanization accelerator in large amounts would cause the surface of the final vulcanized rubber product to suffer from blooming of the vulcanizer and vulcanization accelerator, so that design and hygienic problems would be encountered.
Therefore, it is desired to achieve the developments of an ethylene/.alpha.-olefin/nonconjugated polyene terpolymer rubber composition which enables providing a vulcanized rubber having excellent resistances to heat aging, weather and ozone and whose vulcanization velocity is high as compared with that of the conventional EPDM and a vulcanized rubber obtained from the composition.